Nonaqueous Titrimetric Determination of Ammonium Perchlorate in Double-Base Propellant SIR: Ammonium perchlorate is a major constituent in many double-base slurry and cast propellants and casting powders. As such, a rapid and accurate means of determination is necessary for quality control of both routine production and experimental nitrate ester fuels. Present methods used by propellant manufacturers are time-consuming and of questionable reliability. The technique primarily used in this laboratory for direct measurement of ammonium perchlorate in propellant was based on the Parr bomb reduction of the perchlorate to the chloride and subsequent titration with silver nitrate. The reliability of the procedure was especially affected by incomplete reduction of the perchlorate and/or the presence of chlorate and chloride. An x-ray fluorescence method studied in this laboratory was the only other direct approach known to the authorsi.e., not requiring any separation steps. Another indirect instrumental approach used in Hercules laboratories involved measurement of ammonium perchlorate in the infrared. However, neither instrumental approach has received wide acceptance. Numerous wet chemical methods have been reported for ammonium perchlorate. Procedures have been described, which were based on both gravimetric (5, 7 , 8) as well as acid-base (4, 6, 8) and redox (1) titrimetric determinations of the perchlorate ion. However, none of these methods appeared applicable to propellant without the incorporation of one or more lengthy extractions. Measurement of the ammonium ion as a means of determining ammonium perchlorate has been attempted. The formol reaction (6) was investigated but once more several propellant components, especially nitrocellulose, interfered with the determination. The nonaqueous titration of the ammonium ion with sodium methoxide was reported by Fritz (2); however, nonaqueous titrations with this or other basic titrants have not been used for the direct analysis of ammonium perchlorate in propellant. Investigations by other workers have been made, using sodium methoxide with N,Ai-dimethylformamide as a solvent but two lengthy extractions were necessary for propellant analysis. Preliminary work by the
authors, once more, verified that in basic solvents nitroglycerin, nitrocellulose and several other components interfered in the nonaqueous titration of the ammonium ion. By proper selection of solvent and titrant these interferences were eliminated and the ammonium ion was quantitatively titrated. EXPERIMENTAL
Reagents. All titrants were prepared 0.05N in either reagent grade isopropanol or methanol and standardized against reagent grade ammonium perchlorate. The tetrabutylammonium hydroxide (TBAH) was obtained as a one-normal solution in methanol from Southwest Analytical Laboratories. The cresol red indicator solution was made 0.1% weight by volume in methanol. For synthetic test mixtures, the following materials were used: production grade, Type C 2,4,6,8cyclotetramet h y l e n e t e t r a n i t r a m i n e (HMX) ; Grade A, Type 1 nitrocellulose containing a minimum of 12.6% nitrogen; rocket grade, plant stock, nitroglycerin 99.9% pure; and production grade
aluminum powder of 25- to 30-micron particle size. Two routine production casting powder samples were used for this study. A specially prepared cast propellant, used for interlaboratory analytical studies, was also included. Apparatus. An automatic recording titrator was used with a glasscalomel electrode system. A saturated aqueous potassium chloride solution was used in the calomel electrode. Propellant extraction equipment was described in MIL-STD2868, Method 104.1.2. Procedure. For direct analysis of casting powder and slurry propellant, the samples were ground and a sufficient quantity was weighed to contain 1.5 meq. of ammonium perchlorate. These samples were dissolved in 100 ml. of acetone. After dissolution of solubles, potentiometric titrations were then performed using 0.05N methanolic potassium hydroxide and a glass-calomel electrode pair. Potentiometric standardization of the titrant against ammonium perchlorate in acetone precluded the need for performing blank determinations. Titration speed was not critical. If the samples were
I I
1
I I
19
ml. Figure 1 .
20
0.05 N METHANOLIC KOH
Titration of propellant using various solvents
Samples of about 1 gram of propellant taken to contain 0.1 gram of ammonium perchlorote in a total solvent volume of 100 ml. Curves displaced vertically for illustrative purposes
A. Acetone B. Acetonitrile C. 1 :1 Acetone-isoproponol D. 1:l Acetoncmethonol E. Dimethyl formamide F. Dimethyl sulfoxide VOL. 30, NO. 4, APRIL 1966
a
623
exposed to air for a considerable time, a nitrogen purge was desirable. When analysis of the extract residue for ammonium perchlorate was necessary, the sample was ground and extracted according to MIL-STD-286A, Method 104.1.2. The residue was dissolved in either acetone or acetonitrile (as analysis for other components required), and made to volume. Aliquots were taken to contain about 1.5 meq. of ammonium perchlorate. The titration was performed similarly to the direct analysis except a visual end point detection (using 5 to 10 drops of 0.1% cresol red) could be substituted, colored stabilizers having been removed by the extraction. RESULTS AND DISCUSSION
Solvent Selection. Suitable titration curves of ammonium perchlorate alone were obtained in several solvents with sodium methoxide. With N , N dimethylformamide and dimethylsulfoxide, nitroglycerin, nitrocellulose, and HRlX appeared to compete with the ammonium ion for the titrant and quantitative results were not obtained. When inert solvents, such as acetone or acetonitrile were employed,no interferences in the titration from any propellant components were experienced with the titrants studied. Figure 1 illustrates the effects of several solvents on the titration with methanolic potassium hydroxide of ammonium perchlorate in actual propellant samples. The titrations using N,N-dimethylformamide and dimethylsulfoxide did not give useful potential changes. Those in acetone, acetonitrile, 1: 1 acetone-isopropanol, and l :l acetone-methanol did give sharp end points (up to about 250 mv.) although in the case of the last mentioned, the potential break a t the end point was suppressed. Titrant Selection. The applicability of several basic titrants was investigated using actual samples dissolved in acetone. Figure 2 shows the titrations of eqdal amounts of sample with sodium methoxide, tetra-
Table 1.
I
1
ml. Figure 2.
0.05
624 *
ANALYTICAL CHEMISTRY
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20
Titration of propellant using various titrants
Samples of about 1 gram of propellant taken to contain 0.1 gram of ammonium perchlorate dissolved in 100 ml. of acetone. Curves displaced vertically for illustrative purposes
A.
E. C. D.
NaOCHa in methanol K O H in methanol K O H In isopropanol TBAH In methanol
butylammonium hydroxide, potassium hydroxide, all 0.05N in methanol, and 0.05N isopropanolic potassium hydroxide. The alcoholic potassium hydroxide titrants gave the largest breaks, but the use of isopropanol also resulted in a precipitate which sometimes caused noisy potential response. Therefore, methanolic potassium hydroxide was the selected titrant. Visual end point detection using 0.1% cresol red was suitable with extracted propellant samples. Effects of Water and Carbon Dioxide. The effects on the titration of added water were studied. Up to 5% water in the solvent did not noticeably decrease end point breaks nor affect the accuracy of the determination. At higher levels of added
Reliability Studies
% NH4CIO4, % "4C104, titration on residue direct titration % NH4C104, Potentiometric Visual potentiometric Parr bomb Special check Av. 11.16 11.14 11.08 11.1 sample" 95% c.1. f 0.17 95% c.1. f 0.13 9570 c.1. f 0.06 Casting powder 1 Av. 7.12 7.16 6.95 6.91 9570 C.1. f 0.08 95oJ, C.1. f 0.09 9570 C.1. f 0.10 Casting powder 2 Av. 27.83 27.83 27.76 27.8 9570 c.1. f 0.07 95% C.1. f 0.14 9570 c.1. 0.18 Propellant formulated to contain 11.0% NHaClO4. 0
N TITRANT
I
19
water, recovery was less than 100%. It was noted t h a t the use of an aqueous calomel solution did not affect titration results. I n fact, soaking the electrode system in water between titrations definitely enhanced end point potential breaks. The effects of carbon dioxide on the titration were studied. When sample solutions were allowed to stand open to the air for several hours or more, the resulting titration curves exhibited a slight skewing in the end point region. This was due to absorption of small amounts of carbon dioxide. When the samples were run immediately after dissolution or after a nitrogen purge, sharper and more symmetric potential curves were obtained. Sample Stability. Past the end point, potential drifting and indicator fading were observed with propellant samples in all solvents investigated. The fading was observed even in a closed titration vessel from which carbon dioxide was excluded. This drift in potential was probably due to very slow solvolysis in excess titrant of some propellant components to acid species. This reaction was very slow in acetone and acetonitrile and did not affect analytical results. Recision and Accuracy. A reliability study of the procedure was performed using three propellant formulations, including two production cast propellants. The third material tested was a special propellant formulation for interlaboratory con-
trol testing. The ammonium perchlorate level in these samples ranged from 7 to 30Yc and the sample contained wide variances in ingredients most likely to interfere. This study was performed employing both direct analysis of propellant with potentiometric end point detection and analysis of the extraction residue with both visual and potentiometric end point detection. The results of this reliability study are shown in Table I. Both direct analysis and measurement on extracted samples for ammonium perchlorate agreed closely with the amounts theoretically in the test materials and found by the Parr bomb method. The slight bias noted for rebult~between the direct and extract residue analyses was attributed to error
in titrant standardization. The 95% confidence limits for the three types of test sample as shown in Table I ranged from &0.07% to =t0.18% for ammonium perchlorate in the range from 7 to 28%. No significant difference in precision between the direct analysis and that on the extract residue was found. Thus from this study, the nonaqueous determination of ammonium perchlorate could be equally well performed directly or on the extract residue. -41~0,colorimetric and potentiometric end point detection were equivalent for analysis of the extract. For quality control of present routine type propellant, the direct sample analysis with potentiometic end point detection was suitable.
LITERATURE CITED
(1) Burns, E. +4., Muraca, R. F., ANAL. CHEM.32,1316 (1960). (2) Fritz, J. S., Ibid., 24,306 (1952). (3) Glover, D. J., Rosen, J. >I Ibid., ., 37,306 (1965). ( 4 ) Xorris, M.D., AXIL. CHEM.37, 977 (1965). ( 5 ) Negu, H., Bunseki Kagaku 10, 571 (1961); C.A. 56, 26f(1962). (6) “Treatise on Analytical Chemistry,” 5’01. 5, Part 11, I. AI. Kolthoff, and P. J. Elving, eds., p. 281, Interscience, New York, 1961. ( 7 ) Willard. H. H.. Perkins. L. R.. ANAL. CHEM.25; 1634 (1953). ’ (8) Willard, H. H., Smith, G. RI., IND. ENG.CHEM., h b L . ED. 11, 186 (1939). \
,
R. J. BACZUK R. J. DUBOIS
Hercules Powder Co. Mail Stop 114-B Magna, Utah 84044
Fluorine Resonance Spectra-Structure Correlation for Perhalogenated Propanes SIR: Gross correlations between magnetic resonance positions and chemical structure have been discovered for both proton and fluorine chemical shifts (W, 3 , 6 , 8). Recently, more refined proton resonance correlations have been reported (10, f f ) that reflect changes in chemical shift with small change. in substituents of simple molecules. Because the fluorine resonance shifts of the 2,2-difluoropropane series did not correlate aq well as the proton resonance shifts of the same molecules (11), the fluorine shifts in perhalopropanes have been studied. EXPERIMENTAL
Fluorine resonance spectra of all samples were obtained a t 56.4 hfc. on a Varian Model 4300-B high resolution spectrometer equipped with a field homogeneity control unit. The materials were scanned as 10% by liquid volume solutions in carbon tetrachloride
and, when possible, as pure liquids; in either case, a small amount of fluorotrichloromethane was added to each sample tube for internal fluorine resonance reference ( 5 ) . Chemical shifts and coupling constants were determined by the usual side-band modulation technique ( I ) . RESULTS AND DISCUSSION
Proton resonance chemical shifts vary predictably with varying nonadjacent propane substituent (10); also, proton t o fluorine and fluorine to fluorine coupling constants are dependent upon skeletal substituent electronegativity (9, If). It was hoped that the chemical shift trend indicated in the difluoropropane study (11) might be improved upon by the elimination of one resonating atomic species in the molecule; consequently, the ’9F resonances of a series of chlorofluoro-propanes have been investigated.
Chemical shift values for 10% solutions of the perhalogenated propanes are listed in Table I. Four kinds of C-F group (1 internal and 3 terminal) are to be found in these molecules and Figures 1 and 2 show the correlations derived for the various group resonances indicated in Table I. The correlation shown in Figure 2 does not extend to the molecules of the difluoropropane series (11) ; there is a greater chemical shift per substituent electronegativity sum in the protoncontaining molecules. I n addition, in the difluoropropane series the niethine and methylene end groups have diff ernt effects on the central CF2 group resonance; in these perhalo correlations, however, there is apparently no chemical shift dependence of the central CF, resonance with separation of electrical affinity. Coupling constants are also dependent upon the sum of substituent
Table 1.
Carbon atom no. 1
2
3
Observed NMR Parameters for Perhalopropanes Sum of propane subIgFchemical shift5 stituent electroMid chain Terminal negativities CFZ CF CF2 CFI
25.8 26.7 27.6 28.5 29.4 30.3 31.2
98.07 103.22 108.85 114.02 119.44 125.61 131.47
63.93 66.68 70.97
Coupling constantsb JIZ
J23
JI
a
4.9 64.82 67.91 70.01
80.91 82.95
6.1
